Workflow Baking and Retopo question?
Without watching the video tutorial, a high and low resolution model cannot share the same UV-map therefore geometric details from the higher resolved mesh are baked into a normal- (or other type) map which is unwrapped from the low res model. 


The generated map is used to create details (fake geometry) for lower rendering time which is very common in games.



The workflow in this normal map example :


Unwrap the low res model
add a matrial and texture
select high res and then low res model
bake from selected to active